1. Field of the Invention
The present invention relates to a magnetic tape cassette. Particularly, it relates to a magnetic tape cassette provided with a locking means for locking reels.
2. Description of Prior Art
There has so far been provided in a magnetic tape cassette, especially, in a video tape cassette a locking means for locking rotation of reels held in the tape cassette during non-use, for the purpose of preventing rewinding of the magnetic tape during preservation during non-use and transportation of the tape cassette.
FIG. 1 shows a conventional locking means. In the figure, a pair of reels 5, 7 for a magnetic tape 3 are put in a magnetic tape cassette body which is constituted by an upper half casing (not shown) and a lower half casing 1. The shaft of each of the reels 5, 7 is loosely fitted to a pair of through holes 9, 11 formed in the lower half casing 1 respectively. Each shaft is adapted to be driven by a driving shaft extending upwardly from a magnetic tape recorder. The outer peripheral edges of the lower flanges 13, 15 of the reels 5, 7 are respectively provided with saw-teeth like notches 14, 16. In the central part of the back wall 17 of the lower half casing 1, there are formed a pair of guide pieces 19, 21 between which a guiding channel 23 is formed extending between the front and back of the magnetic tape cassette. In the guiding channel 23, a locking means 27 is fitted with its rear part being pushed forwardly by a spring 25 as shown in FIG. 1 so that when the magnetic tape cassette is mounted on the tape recorder, the locking means 27 is slidingly moved backward. A pair of flexible pawls 29, 31 having elasticity are fitted to, or formed integrally with, the front part of the locking means 27. The flexible pawls are adapted to be normally opened in the directions shown by arrow marks. The opening of the flexible pawls 29, 31 is restricted by means of a pair of pins 33, 35 provided in the front of the guiding channel 23 of the lower half casing 1. When the magnetic tape cassette is not mounted on the tape recorder, the spring 25 pushes the locking means 27 forwardly open the flexible pawls 29, 31 so that the flexible pawls are engaged with the saw-teeth like notches 14, 16 of the lower flanges 13, 15 of the reels 5, 7. When the magnetic tape cassette is mounted on the tape recorder, the locking means is pushed backwardly by an operating means extending from a through hole formed in the bottom surface of the locking means 27 in the lower half casing 1. Backward movement of the locking means releases engagement of the flexible pawls with the reels 5, 7 and the pins 33, 35 close the flexible pawls inwardly.
Magnetic tape cassettes are often left for a long time while they are mounted on tape recorders, on account of which there results thermal deformation in the flexible pawls 29, 31 of the locking means in the closing state under application of stress. Accordingly, when the locking means is returned to the locking position, function of engagement of the flexible pawls to the reels may not be attained. In such cases, there results a trouble that a wound magnetic tape becomes loose and fitting operation of the magnetic tape to a tape running system of a tape recorder can not be performed smoothly.
The present inventors have previously proposed construction of a locking means having reliable function regardless of permanent deformation in the flexible pawls in Japanese Utility Model Application No. 4205/1984 corresponding to copending U.S. patent application Ser. No. 691,891 now U.S. Pat. No. 4,585,187, to eliminate the above-mentioned problem. Namely, the problem has been solved by providing a locking means 39 having a pair of flexible pawls 41, 43, the opposing surfaces of the pawls being provided with cylindrical raised portions 45, while a guide rib 51 having tapered guide surfaces 53, 55 is positioned in front of a pair of pins 33, 35 as shown in FIGS. 2, 3.
FIG. 2 shows a state of the locking means when a magnetic tape cassette is in use (it is mounted on a tape recorder). In this state, even though there results permanent deformation in the flexible pawls 41, 43 by a compressive force of the pins 33, 35, the pawls 41, 43 can be forcibly opened by the contacting of the raised portions 45, 45 with the guide surfaces 53, 55 of the guide rib 51 when the locking means is advanced (when the tape cassette is removed from the tape recorder) whereby the top ends of the pawls 41, 43 can be engaged with the saw-teeth like notches 14, 16 of the reels. In the locking means having the above-mentioned system, however, it has been found that satisfactorily smooth movement of the locking means can not be attained. More specifically, in FIGS. 2 and 3, the flexible pawls 41, 43 are always in contact with pins 33, 35 while the top ends of the pawls and the raised portions 45, 45 are brought into contact with the tapered surfaces 53, 55 at the time of advancing, with the result that compressive forces act on the pawls from the inside and the outside to cause a large frictional force whereby resistance against movement of the locking means becomes large, hence smoothing movement of the locking means is hindered.